Retractor spool

ABSTRACT

Force limitation in retractors is needed to avoid injury to an occupant from the seat belt itself particularly at high crash impact loads. The invention provides a seat belt retractor comprising a spool including force limiting means for allowing further payout of the seat belt webbing after the spool has locked, under the influence of an occupant&#39;s forward momentum in the crash condition. In one aspect the force limiting means is provided by a plurality of fins provided on the outer surface of the spool, each fin having a section which extends in a generally radial direction and a section which extends circumferentially, the circumferentially extending section of one fin overlapping the circumferentially extending section of an adjacent fin and supported by the radial extending section of that adjacent fin, so that a force tending to tighten the webbing on the spool, acts in a direction to crush the radial sections of the fins and to slide the circumferential sections over each other, thus reducing the effective outer circumference of the spool. In another aspect the force limiting means comprises an energy absorbing member connecting the spool to the spool shaft, the connection being rigid below a predetermined force, and above the predetermined force there being relative rotation of the spool with respect to the spool shaft to allow pay-out of belt webbing.

DESCRIPTION

The present invention relates to a retractor spool for a vehicle safetyrestraint.

A retractor spool generally consists of a cylindrical bobbin with acircular cross-section. Seat belt webbing is attached to and woundaround the spool and the spool is mounted on a spool shaft to berotatable in the retractor to wind in webbing under action of aretractor spring and to pay out webbing under the influence ofrelatively gentle forwardly directed inertia of a vehicle occupant, forexample to allow for normal movement associated with vehicle occupancysuch as reaching forwards to activate in-car controls (for a radio or awindow) or to reach a glove compartment or door pocket. In the event ofa crash situation, the more extreme momentum of the occupant activates acrash sensor which locks the spool against rotation and thus preventsforward motion of the occupant and injury due to occupant collision withthe interior fixtures of the vehicle such as the steering wheel,dashboard or windscreen.

However, this sudden locking of the seat belt spool under crashconditions can itself sometimes cause injury to the occupant due tosudden impact of the torso with the belt webbing. This is particularlytrue in high velocity crashes.

In recent years this problem has been recognized and some solutionsproposed.

One solution has been to rely on natural elongation of the webbing underhigh loads, and on the spool film effect (natural tightening of thewebbing wound on the spool under high loads), to produce a load limitingeffect.

Another known proposal has been described in EP 0 297 537 A, whereinforce limiting is effected by interposing a plastically deformablemember between the spool and the innermost winding of the belt webbingor by interposing an elastic member such as a spring rubber member orresin member between an end flange of the spool and the latch plate.This allows a pay-out of webbing in proportion to the inertia of thevehicle occupant at the moment of a crash condition being sensed. Thispay-out lessens the otherwise severe restraining forces on the occupantat high vehicle speeds especially during the initial moments of thecrash. It thus reduces the injurious effects of the seat belt in acrash.

The present invention proposed improved load limiting arrangements forretractors.

According to one aspect of the present invention there is provided aseat belt retractor comprising:

a spool mounted on a spool shaft for rotation in the retractor,

a length of seat belt webbing wound on the spool for retraction orpay-out depending on the rotation direction of the spool;

means for locking the spool shaft against rotation when a crashcondition is sensed;

force limiting means for allowing further pay-out of the seat beltwebbing after the spool shaft has locked, under the influence of anoccupant's forward momentum in the crash condition,

wherein the force limiting means is provided by a plurality of finsprovided on the outer surface of the spool, each fin having a sectionwhich extends in a generally radial direction and a section whichextends circumferentially, the circumferentially extending section ofone fin overlapping the circumferentially extending section of anadjacent fin and are supported by the radial extending section of thatadjacent fin, so that a force tending to tighten the webbing on thespool, acts in a direction to crush the radial sections of the fins andto slide the circumferential sections over each other, thus reducing theeffective outer circumference of the spool.

The circumferentially extending fin parts may be stepped so as toprovide a stop for the sliding movement of the fins and to provide aminimum crushed spool size, i.e. a maximum load limiting force, at whichpay out is allowed.

The fins may be fixed in place, in the undeformed spool, by shearelements. This provides a minimum force at which the load limiting comesinto effect. It also provides a visual indication of when the forcelimiting has been deployed, alerting the owner to effect the necessaryreplacement for safety reasons before further use of the retractor.

The shear elements may be formed of small ribs, pins or clips whichfracture at a predetermined load, and with suitable choice of materialan aural signal may also be generated.

According to a second aspect of the invention the force limiting meanscomprises an energy absorbing member connecting the spool to the spoolshaft, the connection being rigid below a predetermined force, and abovethe predetermined force there being relative rotation of the spool withrespect to the spool shaft to allow pay out of belt webbing.

According to the second aspect of the invention the energy absorbingmember of the force limiting means may comprise a slipping clutchconnecting the spool to the spool shaft co-axial of the spool.

In one embodiment the spool is hollow and the clutch comprises aplurality of interposed clutch plates extending alternately, on the onehand inwardly from the inside surface of the spool casing, and on theother hand outwardly from the spool shaft, within the hollow spool.

In a second embodiment the slipping clutch comprises co-operating clutchplates at one or both ends of the spool shaft by means of which thespool is clamped.

The force limiting load is determined by the slipping torque of theclutch plates and this may be controlled by the position of a nut on thespool shaft acting to apply tension to the clutch, either directly orvia a spring or one or more Belville washers between the nut and theclutch, or an elastically or plastically deformable member.

The clutch plates may be generally planar with surfaces having circularinter-connecting ribs, for example of trapezoidal or sinusoidal profile.Alternatively a cone clutch design as is well known in the art could beused, at one or both ends of the spool shaft.

Preferably a non-circular hole is used to avoid the clutch platesspinning on the shaft.

According to a preferred embodiment of the second aspect the nut islinked to the spool so that as the clutch slips under load limitingconditions, allowing rotation of the spool relative to the shaft, thenthe nut rotates and moves along the shaft, increasing or decreasing thetension on the clutch plates accordingly. This acts as an automatic loadcontrol feedback system.

Another variation of the second embodiment is to use a friction clutchwhereby friction material is arranged between the spool and the clutchplate at one or both ends of the spool. The clutch plate may be acombined locking ratchet or a separate component. The frictioncoefficient of the friction material will control the slipping torqueand thus the limiting load. The material could be keyed, attached orchemically bonded to one of the adjacent surfaces. The flexibility andsurface properties of the material help to compensate for anymanufacturing variations in the adjacent surfaces. A suitable frictionmaterial would be the proprietary material Jurid® 850 or 851.

The friction material may be tensioned by a locking nut on the spoolshaft and/or by washers, for example Belville washers or an elasticallyor plastically deformable member.

According to a third embodiment of this aspect of the invention, theenergy absorbing member of the force limiting means may comprise adeformable member. This may be a crushable or deformable tube mounted onthe rotation shaft of the spool and means for applying a compressiveload to the deformable tube axially with respect to the shaft, when ahigh load is applied to the retractor under crash conditions. The tubemay be mounted on the shaft at one end of the spool, or within a hollowspool.

Preferably the deformable tube is deformable outwardly in a radialdirection under an axial compressive load, for example, one end maysplay radially outward or the middle region of the tube may bulgeoutward.

The compressive load may be applied by a nut abutting the deformabletube and mounted on the spool shaft, and which rotates to apply force tothe deformable tube when a high crash load is detected. It can also beadjusted by the pitch and type (e.g. square or sinusoidal) of thethreading on the bolt and nut.

According to a preferred embodiment the maximum and minimum levels ofload limiting are definable. This may be achieved by a suitable choiceof material, shape and size of the deformable tube.

Preferably a bearing disc is provided to rotate with the spool andsubstantially remove torque from the crushable tube member. This gives amore predictable load limiting curve.

This load limiting arrangement may be used with a single-sided lockingretractor, or with a double-sided locking retractor.

Double-sided locking may be achieved by lock dogs at each end of theretractor connected by a rotatable shaft. Preferably however it isachieved by building in a controlled amount of compliance at themechanical end of the spool and using relative rotation of the spoolrelative to the spool shaft during load limiting to lock the other endof the spool, which for example would be the pretensioner end in aretractor where a pretensioner is fitted.

This avoids the long tie bar otherwise needed to connect two lock dogsfor double-sided locking and improves the webbing capacity of the spool.

The invention is particularly applicable to the load limiting retractorknown as the constant force retractor and described in co-pendingInternational application PCT/US95/15002, publication WO 96/16843.

When a pretensioner is fitted it is preferably to avoid the pretensionerloads being transmitted through the load limiter. This can be achievedusing shear pins connecting the spool to the ratchet at the spoolmechanism end. During load limiting the shear pins break and bring aclutch into engagement for the pretensioner. This ensures that thepretensioning load applied to the belt is less than the load limiterthreshold.

Alternatively, according to the second aspect of the invention thedeformable member may be deformable material contained as an insert inone or more hollow discs mounted between the spool and the shaft,coaxial with the shaft. Each disc can be mounted so that the deformablematerial insert at one side co-operates with an adjacently mounted ballrestrained in a socket. In the spool at rest position and in normal use,the ball sits in a recess in the deformable material. Under loadlimiting conditions, there is relative rotation between the spool andthe shaft and hence between the insert and the ball, and the balldeforms the insert material.

Preferably several discs are provided each having the deformable inserton one side and the ball on the other for co-operating with an adjacentdisc insert. Each disc has a lug for engaging an adjacent disc and theyare held against each other by a nut holding the assembly together.

The innermost disc has an interlocking dog to engage the spool.Compression forces on the spool due to tension in the seat belt webbing,compress the material in one or more discs to take up excess forces. Theviscosity and volume of the deformable material, and also the relativesizes of the chambers can be chosen so as to determine the extent of theload limiting, including minimum and maximum levels.

According to a third aspect the force limiting means comprises anextrudable material contained within one chamber in a hollowcompressible spool having two chambers therein communicating via one ormore holes in a dividing member. Compression forces on the spool due totension in the seat belt webbing, compress the material in one chamberforcing it through the hole or holes in the dividing member, into thesecond chamber. A bursting member may be used to seal the hole or holesto control the initial load. This could be in the form of a foil stickeror stickers in thin section. The viscosity and volume of extrudablematerial, and also the relative sizes of the chambers can be chosen soas to determine the extent of the load limiting including minimum andmaximum levels.

According to a fourth aspect of the present invention the force limitingmeans comprises resilient biasing means acting between the spool and itsaxis of rotation, for example a coiled tension spring wound around aspool shaft coaxial with the spool, or a clock spring or springsoperating in a similar manner to a normal retractor spring.

In such an arrangement there will be an initial preset torque in thespring which determines the minimum load at which limiting starts tooccur. As the load increases beyond this minimum, the spring is wound uptight and the diameter of its centre bore decreases. A maximum forcelimitation occurs when the spring is wound tightly about the spoolshaft.

Alternatively this arrangement could be used with a spring mechanism thecentre bore size of which increases with increasing load up to a maximumdetermined by the size of the internal bore of the spool.

This aspect of the invention is particularly advantageous because theforce limiting means is re-usable and is automatically reset after crashconditions have abated. After impact the restrained occupant reboundsoff the belt into his seat. The taut spring then uncoils, rewindingslack in the belt webbing and satisfactorily restraining the occupant inthe event of a second impact.

For a better understanding and to show how the same may be carried intoeffect, reference will now be made to the accompanying drawings inwhich:

FIG. 1 is a perspective view of a spool for a retractor according to oneembodiment of the present invention;

FIGS. 2 and 3 are cross-sectional views of the spool of FIG. 1respectively before and after a load limiting operation has occurred;

FIG. 4 is a perspective view of a spool for a retractor according to asecond embodiment of the invention;

FIGS. 5 and 6 are cross-sectional views of the spool of FIG. 4respectively before and after a load limiting operation has occurred;

FIG. 7 is a cross-sectional view of a retractor according to a thirdembodiment of the invention;

FIGS. 8 and 9 are cross-sectional views of parts of the retractor ofFIG. 7, taken along lines VIII—VIII and IX—IX respectively;

FIG. 10 is a cross-sectional view of a retractor according to a fourthembodiment of the invention;

FIG. 11 is a cross-section of part of the retractor of FIG. 10, takenalong line XI—XI;

FIG. 12 is a cross-sectional view of a retractor according to a fifthembodiment of the invention;

FIG. 13 is a cross-sectional schematic view of a retractor according toa fifth embodiment of the present invention;

FIGS. 14 and 15 are schematic views to the left (pretensioner) side andthe right (mechanism) side respectively of the retractor of FIG. 13;

FIG. 16 is a graph of force against time for the load limiting retractorof FIGS. 13 to 15;

FIG. 17 shows a sixth embodiment of a load limiting retractor;

FIG. 18 is an enlarged view of part of FIG. 17;

FIG. 19 is a cross-sectional view of a spool for a retractor accordingto a seventh embodiment of the present invention;

FIGS. 20 and 21 are views along lines II—II and III—III respectively ofFIG. 19;

FIG. 22 is an enlarged cross-sectional view of part of the spool of FIG.19;

FIG. 23 is a cross-sectional view of a retractor according to an eighthembodiment of the present invention;

FIG. 24 is an enlarged view of a part of FIG. 23; FIG. 25 is a plan viewof apart of FIG. 23;

FIG. 26 is a cross-sectional view of part of a retractor according tothe invention showing an alternative modification of the eighthembodiment shown in FIG. 23;

FIGS. 27 and 28 are cross-sectional views of a retractor according totwo further modifications of the embodiment of the invention shown inFIG. 23.

Like parts are denoted by like reference numerals throughout.

In FIG. 1 a hollow spool 1 is formed effectively as a double walledcylinder. An inner wall 2 has struts 3 extending radially inwardlytowards the rotation axis or shaft of the spool (not shown). The outerwall 4 has longitudinally extending gaps 5 and is formed as ribsextending generally radially outwardly from the inner wall 2 and angledto provide outer wall portions extending in a directioncircumferentially of the spool. The outer wall portions are stepped tosupport an adjacent wall portion on the circumferential surface thereofwhilst providing clearance between adjacent ribs at the gaps.

FIG. 2 is a cross-section of the spool of FIG. 1 under normal operatingconditions. FIG. 3 shows the spool under loaded conditions, e.g. duringa crash situation.

As can be seen from a comparison of FIGS. 2 and 3, under loadedconditions, as the seat belt is tightened, it compresses the spool 1 andthe circumferential parts of the stepped ribs slide over each otheruntil their ends reach the stops provided the adjacent step. This is thecondition shown in FIG. 3 where the spool diameter is reduced under theload, resulting in a larger pay-out of webbing than would otherwiseoccur, thus limiting the force transferred to the vehicle occupant bythe restraining webbing. In this condition the gaps 5 in the outercircumferential wall or layer are closed defining a maximum forcelimitation and preventing further collapse of the spool diameter andfurther spool pay-out.

The embodiment of FIGS. 4 to 6 is an improvement on that of FIGS. 1 to3, incorporating small fracture pins 6. These fracture at apredetermined load and define the minimum force at which the forcelimitation is engaged.

In FIGS. 7 to 9 the force limitation is by way of a slipping clutch 7between the spool 1 and the spool shaft 10. This is formed ofalternating clutch plates 8 and 9 (shown in plan view in FIGS. 8 and 9respectively). The plates are forced together by Belville washers 11tensioned by a nut 12 threaded onto the shaft 10. A spring or simply thetorque of the nut or a crush bush could replace or be used incombination with the washers to control the force applied to the clutchplates and thus the limiting load.

In an alternative embodiment (not shown) the nut is linked to the spoolso that as it rotates relative to the shaft adjusting the clutchpressure, it increases or decreases the limiting load and acts as a loadcontrol feedback system.

In FIG. 10 the spool 1 has a hollow centre which comprises two chambers13 and 14 separated by a perforated plate 15. An extrusion material ispacked into one chamber 13 filling the space therein between the spoolshaft 10 and the spool wall 1.

Under conditions where load limiting is required, when the seat beltwebbing wound on the outside of the spool 1 is tightened, spool 1 iscompressed, and the size of each of the chambers is decreased. Thiscauses extrusion material 13 to be forced through the perforations 16 inthe dividing disc 15, into the second chamber 14. The extent of the loadlimiting is determined by the viscosity of the extrusion material, bythe size of the perforations and by the relative sizes of the chambers.

In FIG. 12 a torsion spring 17 is coiled around the spool shaft 10within the hollow of spool 1. It is fixed at its ends 18 to the spooland at its middle to shaft 10 by pin or bolt 19.

In a crash impact the forward momentum of the occupant being restrainedacts on the spool 1 to try to pay out more belt webbing. Since the shaft10 is locked by a locking mechanism, activated by a crash sensor, thenthe relative rotation of the spool to the shaft tightens the spring 17as more webbing is paid out, until the spring is coiled tightly aroundthe shaft at which point pay-out ceases since the spool 1 is locked tothe locked shaft 10.

When webbing tension is released as the crash conditions abate, thespring acts to rotate the spool in a rewinding direction thus drawing inthe extra payed-out webbing together with any slack (which may have beencaused by webbing stretch or by cinching of webbing on the spool). Thusthe occupant is well protected against a second impact.

If the spring 17 is wound the opposite way then, as webbing is paid outin a load limiting operation, the spring will expand until it fills thespool 1, which defines therefore the maximum force at which loadlimiting operates. The spring will draw in webbing as it subsequentlyrecoils itself.

FIGS. 13 to 15 show a retractor with spool 1 on which is wound beltwebbing 22 and which is rotatably mounted on a shaft 10. A load limitermember in the form of a deformable hollow cylindrical tube 24 is mountedon shaft 10 between a bearing disc 25 and a nut 26 which is also mountedabout shaft 10.

Rotation of the spool after it is locked, to prevent further webbing payout in response to a crash sensor, causes nut 26 to rotate on the shaft10 and move towards the hollow tube 24, thus compressing it between thebearing disc 25 and the nut 26. In FIG. 1 the tube 24 is shown in acompressed form being bulbous. The bearing disc 25 rotates with thespool and thus removes all torque from the crushable member, giving amore predictable performance.

FIG. 15 illustrates a main ratchet wheel 27 at the right hand (mechanismend) of the spool 1. Teeth 28 on the outer circumference of the mainratchet wheel 27 are engaged by a locking pawl when a crash situation isindicated to lock the right hand side of the spool 1. Wheel spokes 29are constructed to deform under a predetermined load as shown in FIG.15a, allowing torque to be transferred to the other end (spring side) ofthe spool 1, allowing locking teeth at the other end to engage and sharethe locking torque.

FIG. 14 illustrates the spring side of the spool 1 and shows spacedlocking bars 30 mounted in carrier plate 31. During load limiting therelative motion between the spool 1 and the lockbar carrier plate 31pushes the lockbars 30 radially outwardly to engage teeth on theretractor frame (not shown). This locks the left hand side of the spool1 and thus the spool is locked securely at both sides after loadlimiting.

The sequence of operation is shown by the graph of FIG. 16 where line Irepresents the load on the main ratchet (mechanism side) and line IIthat on the secondary ratchet (spring side).

At the onset of the crash situation, the spool 1 is locked at A. As theload on the webbing increases due to the inertia of the vehicleoccupant, the webbing is pulled tighter on the spool. This is known ascinching and is shown on the graph at B. When the webbing is fullytightened on the spool, the load transfers to the spool; and the loadlimiter comes into effect at C taking all the load on the main ratchet.At D the load limiter bottoms out and the main ratchet yieldstransferring some load to the other end of the retractor. The load onthe secondary ratchet thus increases at this stage while that on themain ratchet remains fairly constant at E.

FIG. 17 shows an alternative load limiting member in the form of ahollow conical tube 24. Like parts are denoted by like referencenumerals and the function of the arrangement is identical to that forthe arrangement of FIGS. 13 to 15. The hollow conical tube 24 expandsradially when an axial force is applied. The shape of the conical tube24 of FIG. 17 is shown in greater detail in FIG. 18.

The sequence of operation of this arrangement with a pretensioner willnow be described.

When pretensioning begins, the spool 1 and the mechanism end mainratchet 27 are connected by shear pins and the ratchet 27 is locked byengagement with a lockbar (not shown).

As the pretensioner force increases, the shear pins break and the loadlimiter operates. All the load is at the mechanism end on the mainratchet 27 and the spring side (left hand side in FIG. 1) lockbars 30are not engaged with the frame although they are driven outwards towardsthe frame.

When load limiting ceases the load on the spool increases causingdeformation of the spokes 29 of the main ratchet 27. This allows thespring end locking bars 30 to contact the frame thus completingdouble-sided locking with a controlled sharing of the load between thetwo ends of the spool.

The construction of the spring side locking system (locking bars 30) canbe varied to either maintain the spool locked after the pretensioningstroke, or to release it to run freely, as desired.

FIG. 19 shows a hollow retractor spool 1 mounted for rotation on a spoolshaft 10 supported in bushings 31 in a housing frame 32.

Multiple discs 33 are mounted on the shaft 10 within the webbing spool 1of a seat belt retractor mechanism.

This shaft 10 has the retractor's locking gear/ratchet permanentlyaffixed. The discs 33 each have an insert of deformable material and aball. The webbing spool is mounted on this shaft with the innermost disc33′ engaged to the spool 1 by locking dogs 35. The whole is heldtogether by a nut 34. The nut thread is handed such that when the spoolis under load the nut 34 will tend to tighten on the shaft.

The innermost disc 33′ has locking dogs in one face to engage with thedogs 35 in the bore of the spool 1. The other face of innermost disc 33′accommodates a ball 38 from the adjacent disc. The face of each discalso has a raised lug (not shown) to engage in a circular slot in theadjacent disc, to hold the discs together. This slot is around the discfrom the outside of the walls making up the socket for the ball. Eachintermediate disc has a socket in one face for a ball 38 which protrudesto engage in the recess 37 in deformable material 36 in an adjacent disc33. The shaft 10 has a similar socket for the ball and circular slot.The socket and slot features may be integral with the structure of theshaft 10 or may be part of a separate disc permanently affixed to theshaft 10. On operation of the retractor under crash conditions the shaft10 is stopped from rotation by the retractor's normal locking mechanism.This new design allows the spool 10 to continue a loaded rotation. Thespool 10 being engaged on the locking dogs of the inner disc and by theballs in the recesses of the discs' deformable material causes all thediscs to rotate. Under the load induced the discs will tend to pushapart and thus push the spool 30 up against the nut 31, thus tending totighten the nut. If all the discs 33 have a similar insert material,then material in the intermediate disc nearest the locking gear/ratchetside (right hand side) of the retractor will start to be deformed by theball in the shaft. The deformation will continue until the lug on thisdisc stops at the end of the circular slot in the shaft. At this pointthe material in the next intermediate disc starts to deform as in theprevious disc and so on until the lug on the inner dog disc 33′ comes toa stop. The spool 10 then ceases rotation. Alternatively with all discshaving the same insert material they may all start to deform at the sametime until full rotation of the spool is achieved. This design allowsfor the number of discs 33 to be varied according to the requiredrotation of the spool and also allows for varying loads by using adifferent insert material(s). If more than one type of insert materialis used in the same assembly, then the discs with the softer materialwill deform first.

In FIG. 22 an enlarged view is shown illustrating the discs 33 on theshaft 10 more closely.

In FIGS. 23 to 28 the force limitation is by way of slipping clutchplates 38 and 39 acting on transverse flange ends of the spool 1. Theplates are forced together by a Belville washer 52 tensioned by a nut 62threaded onto the shaft 10. A suitable Belville washer would be madefrom fairly hardened conical steel and be about 7.5 to 10 mm thick. Thiswould deform around 1 mm under the large loads involved since theseforces can rise to 2 tons-force (about 20,000 N). A spring or simply thetorque of the nut could replace the washers to control the force appliedto the clutch plates and thus the limiting load. A friction material 43,such as Jurid® 850 or 851 is shown in FIGS. 5, 9 and 10 interposedbetween the spool flange ends and the clutch plates 8 and 9.

In an alternative embodiment (not shown) the nut is linked to the spoolso that as it rotates relative to the shaft adjusting the clutchpressure, it increases or decreases the limiting load and acts as a loadcontrol feedback system.

The clutch plate 38 has circular ribs 40 (FIG. 24) which interlock withribs 41 on the spool 1 end surface. These ribs may be circular,trapezoidal or of other profiles. Both clutch plates 38 and 39 havenon-circular central holes 42 (FIG. 7) for the shaft 10. This helps tostop the clutch plates spinning on the shaft 10.

An alternative clutch is shown in FIG. 26 using a cone clutch 38 a ateither or both ends and which otherwise operates in the same way as theclutch of FIG. 23.

FIGS. 27 and 28 show a friction clutch with friction material 43sandwiched between one end of the spool 1 and the clutch plate 39 (whichmay be a combined locking ratchet and clutch plates or a separatecomponent), and wherein the Belville washer 52 and threaded nut 62 arelocated on the spool shaft 10 within the body of the hollow spool 1.

The friction force is controlled by the tension in the Belville washer52 which is adjusted by nut 62 threaded on the shaft 30. The frictionmaterial will be chosen to optimise the required force limitation andmay be Jurid 850 or 851. The friction can be further improved by keying,attaching or chemically bonding the friction material to one of theadjoining surfaces.

What is claimed is:
 1. A seat belt retractor comprising a spool mountedon a spool shaft for rotation in the retractor, a length of seat beltwebbing wound on the spool for retraction or pay-out depending on therotation direction of the spool; means for locking the spool shaftagainst rotation when a crash condition is sensed; force limiting meansfor allowing further pay-out of the seat belt webbing after the spoolshaft has locked, under the influence of an occupant's forward momentumin the crash condition, wherein the force limiting means is provided bya plurality of fins provided on the outer surface of the spool, each finhaving a section which extends in a generally radial direction and asection which extends circumferentially, the circumferentially extendingsection of one fin overlapping the circumferentially extending sectionof an adjacent fin and are supported by the radial extending section ofthat adjacent fin, so that a force tending to tighten the webbing on thespool, acts in a direction to crush the radial sections of the fins andto slide the circumferential sections over each other, thus reducing theeffective outer circumference of the spool.
 2. A seat belt retractoraccording to claim 1 wherein the circumferentially extending fin partsare stepped so as to provide a predetermined stop for the slidingmovement of the fins which provides for a minimum crushed spool size,and determines a maximum load limiting force at which pay out isallowed.
 3. A seat belt retractor according to claim 1 wherein the finsare fixed in place relative to each other, in the undeformed spool, byshear elements, which shear elements determine a minimum force at whichthe load limiting comes into effect and provide a visual indicationdeployment.
 4. A seat belt retractor according to claim 3 wherein theshear elements are formed of one of small ribs, pins and clips whichfracture at a predetermined load which determines the minimum force.